Dirt-repellent cleaning scraper

ABSTRACT

A cleaning scraper for cleaning roller surfaces in paper machines, wherein the cleaning scraper has a main element and a coating covering at least part of the surface of the main element. The coating is formed with a first coating layer and a second coating layer arranged on the first coating layer. The second coating layer is formed by a polymer.

The invention relates to scraper blades for machines for paper production and relates in particular to the design of cleaning scrapers for cleaning roller surfaces in paper production.

Cleaning scrapers are used in paper machines for cleaning the surfaces of rollers. Cleaning of a roller surface is effected by detaching the contaminants by means of one or more cleaning scraper blades pressing against the surface of the respective coating of the roller. The contaminants, which are usually formed by paper residues, paper fibers and particles, lead to local stressing peaks at the blades, so that cleaning scrapers are usually made of steel or under corrosive use conditions of stainless steel and have a cemented hard material coating at the region provided for contact with the roller in order to achieve a higher operating life or running time.

To produce the cemented hard material coating, it is possible to use a conventional cemented carbide powder comprising, for example, 8-10% of cobalt and tungsten monocarbide as hard material. Coating is preferably carried out using thermal spraying techniques, with the coating material frequently being applied in a plurality of passes in order to achieve a very homogeneous, in terms of its composition and physical properties, cemented hard material coating. Each of the passes applies a thin layer of coating material to the substrate material of the blade or to the last coating layer applied thereto. Application of the coating in a plurality of thin layers ensures that the components of the coating material cannot demix during buildup of the coating. When the coating parameters are observed, a macroscopically homogeneous coating can be produced because of the chemical and physical identity of the individual layers.

However, the coating surface has a certain ability to adhere to the contaminants detached from the roller coating, as a result of which the particles of dirt are removed with increasingly poorer effectiveness by the cleaning scraper and the cleaning action of the cleaning scraper becomes impaired over time.

It is therefore desirable to provide a cleaning scraper which has an improved long-term cleaning action.

In one embodiment, such a cleaning scraper for cleaning roller surfaces in paper machines has a main element and a coating covering at least part of the surface of the main element. The coating here comprises a first metallic coating layer and a second coating layer arranged on the first coating layer, with the second coating layer being formed by a polymer.

A cleaning scraper according to this embodiment presses with the second coating layer formed by a polymer against the respective roller surface. Since the adhesion of contaminants to the polymer is lower than to the metallic coating layer, improved removal of the contaminants detached from the roller surface and thus improved quality of cleaning are achieved. Apart from this dirt-repellent action, the cleaning scraper has improved sliding properties at the contact surface to the roller, as a result of which the forces acting on the coating are reduced and the operating life of the cleaning scraper is thus increased.

It may be pointed out that the terms such as “comprise”, “have”, “include”, “contain” and “with” used in this description and the claims for listing features and also their grammatical derivatives, should generally be interpreted as nonexhaustive listing of features, e.g. components, process steps, devices, ranges, sizes and the like, and in no way exclude the presence of other or additional features or groups of other or additional features.

In embodiments, the polymer comprises an epoxy resin since in the uncross linked or partially cross linked state this readily wets the metallic first coating material and thus adheres well to the latter. In further embodiments, the polymer advantageously comprises a silicone-polyester resin since this combines an excellent dirt-repellent action with very good antiadhesion and sliding properties. To improve the dirt-repellent action and also the sliding properties of the coating surface, fillers can be embedded in the polymer; in preferred embodiments, the fillers contain polyfluoroethylene (PFE) and in particular polytetrafluoroethylene (PTFE). In advantageous embodiments, the fillers are present in the form of particles and in particular in the form of particles having average diameters in the range from 0.1 to 5 μm. Instead of an epoxy resin or silicone-polyester resin having fillers comprising PFE or PTFE, the polymer can also be formed directly by polyfluoroethylene and in particular polytetrafluoroethylene or by a polymer which comprises such a material.

The material of the first coating layer comprises, in embodiments, a cemented hard material which combines good mechanical strength with high abrasion resistance and can be applied effectively and economically to the main element of the cleaning scraper by means of modern thermal spraying processes. In embodiments, the cemented hard material preferably comprises from 8 to 10% of cobalt and contains tungsten monocarbide as hard material. Other embodiments have a first coating layer whose material comprises an oxide ceramic. The oxide ceramic or oxide ceramics of the first coating layer are preferably selected here from among aluminum oxide, zirconium oxide, magnesium oxide, chromium(III) oxide, yttrium oxide and titanates.

To achieve an adhesive bond between first coating layer and second coating layer, the first coating layer preferably has a surface roughness at the boundary to the second coating layer in the range from 5 to 8 μm and in particular in the range from 6 to 7 μm. In further embodiments, a bonding layer is arranged on the surface region of the main element which adjoins the first coating layer so as to improve the adhesion of the first coating layer to the main element. The layer thickness of the first coating layer is preferably in the range from about 50 to 100 μm, while the second coating layer preferably has a thickness in the range from about 20 to 80 μm.

Further features of the invention can be derived from the following description of examples in conjunction with the claims and the accompanying figure. It may be pointed out the invention is not restricted to the embodiments of the examples described but is determined by the scope of the accompanying claims. In particular, in embodiments according to the invention, the features indicated in the examples described below can be realized in numbers and combinations deviating from the examples.

In the following description of some examples of the invention, reference is made to the accompanying FIG. 1 which shows a schematic cross section through a cleaning scraper in the region of its facet.

The schematic depiction in FIG. 1 shows a cross section through the front region of a cleaning scraper 10 provided with a coating, where this front region is provided for contact with a roller surface. The cleaning scraper blade 10 has a main element 11, which can be made, for example, of steel, preferably of stainless steel, and a coating built up of a first coating layer 12 and a second coating layer 13 arranged thereon. At the interface to the first coating layer 12, the main element 11 can, as illustrated in the figure, have a bonding layer 14. The coating occupies at least part of the cleaning scraper 10 which comes into contact with the roller surface of the paper machine. The cleaning scraper 10 also has a chamfer, which is generally referred to as facet, in the front region provided for contact with a roller surface. In general, the coating covers the main element 11 in the region of the facet, as shown in the figure.

As can be seen from the schematic detail A of the coating in the region of the facet, the coating has two coating layers 12 and 13, with the first coating layer 12 arranged on the main element preferably being formed by a metallic material which advantageously consists of a cemented hard material or comprises such a material. The material of the second coating layer 13 arranged on the first coating layer 12 is a polymer.

In preferred embodiments, a thermal spraying process is used for producing the first coating layer 12. Spray coating is preferably effected in a plurality of passes, for example from 10 to 100 passes. Each pass produces a thin sublayer of the coating material, with the first sub layer being sprayed directly on the surface of the main element 11 or any bonding layer 14 previously applied thereto and further sublayers being sprayed onto the sub layer previously applied in each case. The physical homogeneity of the first coating layer 12 can be adjusted via the parameters of the process used. In the spraying process designated by the acronym HVOF (high velocity oxygen fuel), it is possible, for example, to avoid the formation of pores in the sublayers by selection of a suitable ratio of fuel to oxygen and a suitable transport rate of the powder material used for layer formation.

As material for forming the first coating layer, it is possible to use commercial cemented carbide powders comprising about 8-10% of cobalt and tungsten monocarbide as hard material. The cemented hard material powder is preferably applied in such a way that a surface roughness of the first coating layer 12 in the range from about 5 to 8 μm and in particular in the range from 6 to 7 μm is obtained. The coating is preferably applied to only part of the main element and, in preferred embodiments, covers an about 20 mm wide region which adjoins the front edge of the cleaning scraper and covers the facet.

In principle, ceramic materials such as aluminum oxide, zirconium dioxide, magnesium oxide, chromium(III) oxide, yttrium oxide and titanates can also be used instead of metallic materials for forming the first coating layer. These too can be applied by means of thermal spraying processes to the main element of the cleaning scraper blade 10. Adhesion of ceramic coatings to the main element 11 is usually ensured in the case of steel substrates by use of a bonding layer whose material can, for example, be selected from among aluminum, nickel, chromium or alloys such as AlNi or NiCr.

After application of the first coating layer 12, a polymer is applied as second coating layer 13 to the surface thereof. The polymer fills the depressions formed by the surface roughness of the first coating layer 12, as a result of which a physical locking of the two coating layers is obtained in addition to an adhesive bond.

Suitable polymers are thermo sets and thermoplastics, which can be produced on the basis of one-component and two-component systems. Thermo sets whose decomposition temperature is so far above the use temperature of the second coating layer 13 that the thermo set behaves elastically are particularly suitable. In an analogous way, thermoplastics whose glass transition temperature is so far above the use temperature of the second coating layer 13 that no troublesome softening of the polymer can occur during use of the polymer-coated cleaning scraper 10 are suitable. A rough guideline value for the minimum difference between use temperature and decomposition or glass transition temperature is 20° C. Use temperature is understood as meaning the operating temperature of the second coating layer 13 during correct use of the cleaning scraper blade 10.

Suitable polymers are, in particular, epoxy resins and epoxy resins comprising filler particles, for example particles of a polyfluoroethylene (PFE) and in particular of polytetrafluoroethylene (PTFE), embedded therein. Since epoxy resin in the uncross linked or partially cross linked state displays good wetting of the first coating layer 12, it has good adhesion thereto.

The viscosity of the epoxy resin can be reduced by addition of solvents, for example alcohols or ketones, in order, for example, to make spraying-on of the polymer possible. In the case of relatively high viscosities, application of the polymer by means of painting tools such as brushes or blades is preferred. When a dipping bath is used, coating of the surface of the first coating layer can also be carried out by dipping into a still liquid polymer.

In a preferred embodiment, a mixture of epoxy resin and isobutanol containing PTFE particles having average diameters of from 0.1 to 5 μm is used as polymer starting material. The proportions of the epoxy resin in the polymer starting material are in the range from 40 to 70% by weight, those of the isobutanol are in the range from 10 to 60% by weight and those of the PTFE particles are in the range from 2 to 20% by weight. The polymer/solvent mixture is preferably sprayed onto the first coating layer.

Another preferred embodiment differs from that described above in the choice of the polymer starting material, using a silicone-polyester resin mixed with isobutanol and containing PTFE particles having average sizes of from 0.1 to 5 μm. The proportions of the silicone-polyester resin in the polymer starting material are once again in the range from 40 to 70% by weight, those of the isobutanol are in the range from 10 to 60% by weight and those of the PTFE particles are in the range from 2 to 20% by weight. The polymer/solvent mixture is preferably sprayed onto the first coating layer.

After application of the polymer based material to the first coating layer 12, it is preferably thermally cross linked, which concludes the formation of the polymeric second coating layer 13.

Instead of or in combination with the above-described polymer materials, it is also possible to use silicones for forming the second coating layer 13. Polymer starting materials having a low viscosity are preferably used for forming comparatively thin second coating layers, for example for thicknesses of from 20 to about 50 μm, while polymer starting materials having a relatively high viscosity are used for correspondingly thicker second coating layers of, for example, up to 80 μm.

The polymeric surface of the cleaning scraper coating reduces the adhesion of dirt on the cleaning scraper, so that this dirt can be more easily removed from the cleaning scraper blade 10 and does not accumulate there. This significantly improves the cleaning action of the cleaning scraper 10 compared to conventional designs. Since the polymeric surface also reduces the friction against the roller surface, the improved cleaning action is also retained over a longer period of time. Furthermore, owing to the different optical properties of first and second coating layers, damage to the coating or an end to the running time of the cleaning scraper characterized by wear of the second coating layer can be recognized more easily and more quickly than in the case of conventional cleaning scrapers. 

1-14. (canceled)
 15. A cleaning scraper for cleaning roller surfaces in paper machines, the cleaning scraper comprising: a main element with a surface; a coating covering at least a portion of said surface of said main element, said coating including a first coating layer and a second coating layer formed of a polymer disposed on said first coating layer.
 16. The cleaning scraper according to claim 15, wherein said polymer comprises an epoxy resin.
 17. The cleaning scraper according to claim 15, wherein said polymer comprises a silicone-polyester resin.
 18. The cleaning scraper according to claim 15, which comprises fillers embedded in said polymer.
 19. The cleaning scraper according to claim 18, wherein said fillers contain polyfluoroethylene.
 20. The cleaning scraper according to claim 18, wherein said fillers contain polytetrafluoroethylene.
 21. The cleaning scraper according to claim 18, wherein said fillers are present in the form of particles.
 22. The cleaning scraper according to claim 21, wherein said particles have average diameters in a range from 0.1 to 5 μm.
 23. The cleaning scraper according to claim 15, wherein said polymer comprises polyfluoroethylene.
 24. The cleaning scraper according to claim 15, wherein said polymer comprises polytetrafluoroethylene.
 25. The cleaning scraper according to claim 15, wherein said polymer comprises a silicone.
 26. The cleaning scraper according to claim 15, wherein a material of said first coating layer comprises hard metal.
 27. The cleaning scraper according to claim 26, wherein said hard metal contains 8 to 10% of cobalt and tungsten monocarbide.
 28. The cleaning scraper according to claim 15, wherein a material of said first coating layer comprises an oxide ceramic.
 29. The cleaning scraper according to claim 28, wherein said oxide ceramic is selected from the group consisting of aluminum oxide, zirconium oxide, magnesium oxide, chromium(III) oxide, yttrium oxide and the titanates.
 30. The cleaning scraper according to claim 25, wherein said first coating layer has a surface roughness in a range from 5 to 8 μm.
 31. The cleaning scraper according to claim 30, wherein said first coating layer has a surface roughness in a range from 6 to 7 μm.
 32. The cleaning scraper according to claim 26, wherein said first coating layer has a surface roughness in a range from 5 to 8 μm.
 33. The cleaning scraper according to claim 32, wherein said first coating layer has a surface roughness in a range from 6 to 7 μm.
 34. The cleaning scraper according to claim 15, which comprises a bonding layer formed on a surface region of said main element that adjoins said first coating layer. 